Air horn



June 2, 1936. R. M. CRITCHFIELD 2,042,881

AIR HORN Filed Novf'l, 1950 l N V EN T0 R 1705:? M C'mrawao BY 24 0 wwaa ATTORN EY5 Fatented June 2, 1936 NITED' AIR HORN Robert M. Oritchfield, Anderson, Ind., assignor,

by mesne assignments, to General Motors Corporation, Detroit, Mich., a corporation of Delaware . Application November 7, 1930, Serial No. 494,061

26 Claims. (01. 1777) This invention relates to sound signals of the air tone type, and has for one of its objects to produce a smoother note, or one of purer air tone, or a tone more closely simulating the tone of a wind instrument and with less mechanical noise.

A further object of the invention is to improve sound signals of the vibratory diaphragm type, having in main to eliminate objectionable noise of operation. 7

Another object of the invention is to provide mechanism for generating sound of the air tone type, by means operating to set an air column into audible vibration, by successively and rapidly exhausting and filling an air chamber with which the air column is normally in communication.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein a preferred embodiment of one form of the present invention is clearly shown.

In the drawing:

Fig. 1 illustrates in part section, a sounding signal embodying the instant invention, and is a view substantially as indicated by the line and arrows |--l of Fig. 2.

Fig. 2 is a rear elevation of the power plant with a section through the enclosing cover, substantially as indicated by the line and arrows 22 of Fig. 1.

Fig. 3 is a modified showing of certain portions of the invention illustrating the association with the power plant device, substantially at right angles to the View illustrated in Fig. 1, and substantially as indicated by the line and arrows 3-3 of Fig. 2.

The above indicated objects with others are accomplished in main by mounting a motor with a vibratory diaphragm in very close relation to a trumpet supporting plate or collar, so as to provide a substantially collapsible chamber bounded by the collar and diaphragm, the sole outlet of the chamber being through an aperture in the plate leading to the trumpet. With the diaphragm and apertured collar in that relation, that is, relatively closely spaced relation, the diaphragm, when actuated, will be moved to and from the collar, and in effect rapidly exhaust and flush or fill the chamber through the collar aperture. The rapid succession of exhausting and flushing the chamber is the result of-diaphragm movement in vibrating through its amplitude, in one direction practically squeezing all of the air, or substantially all of the air, out from between the collar and diaphragm through the trumpet, and in the other direction pulling air into, or tending to allow air to rush back into the chamber through the trumpet due to atmospheric pressure on the outside.

The ideal situation is one in which substantially all, or a very large percent, of the volumetric content of the chamber is expelled at the conclusion of the compression stroke, and in which, during the reverse stroke of the diaphragm a correspondingly large percent of air is drawn into the chamber, both without the attendance of any objectionable mechanical noise of the moving parts due to motor actuation. To accomplish this end, the diaphragm should be set in its normal position relatively close to the trumpet mouth but not so close as to cause any striking of the parts, or to cause suflicient engagement of one part with another to produce an audible vibration of any one of the parts during actuation of the device. It has been found that horns constructed in accordance with the present invention produce pleasing musical notes similar to that of a wind instrument, and having carrying power sufficient for a Warning signal. Quite obviously, the length of the trumpet should be such that the air column it encloses will have a fundamental frequency substantially the same as the frequency of the diaphragm and moving parts of the power plant. It is believed that the foregoing results are obtained because the air in the collapsing chamber between the diaphragm and collar is compressed rapidly to a. relatively high degree, thereby causing the air to move out rapidly from the collapsing chamber during the compression stroke, and then move rapidly into the collapsing chamber during the reverse or intake stroke, thereby causing a vigorous vibration of air in the air column. At the end of the compression stroke of the dia phragm, the diaphragm has been brought into relatively close proximity to the adjacent surface of the horn collar thereby stopping ofl the air column in the trumpet.

With particular reference to the drawing, the several objects of the invention are accomplished by providing a magnet motor 20 to which is secured a diaphragm 40 as by means of a collar 50, that operates to support a projector or trumpet 60 as will hereinafter appear. In each form of the invention illustrated, the power plant is essentially-the same and comprises in main a foundation member 2| provided with means for supporting a field frame 22, as by the long threaded studs 23 engaging supporting ledges 2G and 25,

' as is usual in structures of this type, and well' known to those e'xperiencedin the art. The field frame is made up of E-shaped laminations riveted together so as to provide recesses 26 within which is secured a field winding 21, having appropriate connection with'a source of energyand with a circuitbreaker mechanism 28 carried by a bracket 29 joined to the foundation member 2|.

The circuit breaker mechanism includesa pair 19 -of cooperating contacts-29a'and-3B,-the latter of which is -mounted upon a resilient leaf 31!, the

end overlying an engagement piece 32 carried by an. armature rod 33 slidable .through a bearfing sleeve 34 fixed in a central aperture 35 of the field frame-22. The armature rod 33, after passing through the field frame 22, supports an armature 36 that is separated from the dia-- phragm All by one or more spacers 31', and a clamping disc 38. The rod thence, after pass- 'ing through the diaphragm 40, is' fitted with a second clamping disc as that is followed by a nut and washer on the threaded portion 33 of the armature rod which operates to secure them in rigid relation. r I [While there has been shown and specifically described .a magnet motor or power plant in which the armature, when actuated by the magnet windin pulls the diaphragm away from the collar, this is by no means to be interpreted as a limitation, but rather as a general illustrated embodiment.

Applicant" means also to include that type of magnet motor, in which energization of the magnet winding actuates thearmature to thrust the diaphragm toward the collar. Magnet motors of that type are also more or less conventional in the, art, the patent to Critchfield 1,749,325 of March 4, 1930, being illustrative.

The diaphragm 43 is preferably corrugated as illustrated in Fig. 1, or is otherwise formed or drawn, so as to produce a relatively rigid mem her for the mass involved, and this tends to reduce the inertia of the member and enables one to increase the air movement by means of a dia- 45 phragmof definite-diameter. The formation in this fashion, renders the central and proximate portions ofgreater rigidity, than the more remote portions near the diaphragm periphery, and thus causes the diaphragm, when'vibrated,

so to actfmore in the nature of a piston or dash;

pot. These diaphragm formations may. involve a central flattened portion il adjoiningan an- I nular rib 42 that recurves into an oppositely disposed rib. d3. of larger diameter, and which merges into a flattened peripheral portion 56' that makes access for securing to the foundation member 2| in any preferred manner. 'In the illustrated'embodiment, the diaphragm is secured to the power plant .20 by means of clamping screws-spaced at intervals about the meeting portion of the diaphragm and powerplant as illustrated-in Fig. 2,'and som'e of which cooperate with apertured ears' 46 of the power plant housing 41 to retain the same thereon as '65 is also illustrated in Fig. 2.

'As an aid, to securing the diaphragm fill to thepower plant the collar 50, hereinbefore referred to is appropriately apertured to receive the clamping screws 45 and is' clamped. against the diaphragm by the agency of said screws and their associated nuts. The collar 50 rolls in wardly' to a central or axial boss 5|, that is externally threaded to receive a clampingnut 5g cammed against a collar 53 secured to the toe 54 of thetrumpet Bil, the boss SE of the collar being axially bored at 55 to-provide continuity of passage with'the interior of the trumpet, and

with the trumpet defines an air column of predetermined length. It is well known. to those skilled in this art that the length of the air column should be such that itSv natural frequency is the same as that of the diaphragm and armature .asembly. The .patent to Hutchison 1,111,463, September 22, 1914 is onedisclosu're of that principle. The collar 50, aside from the features just mentioned, has special provisions for closely fitting the surface contour of the diaphragm 40, and is so fashioned that the diaphragmis peripherally engaged by the collar through its clamping function to rigidly secure the same to the power plant. This peripheral engagement between the collar and diaphragm occurs at the annularring portion 56, and from thence the contiguous surface of the.c'ol1ar is similarly formed with respect to the diaphragm so as to provide a space therebetween of increasing axial magnitude inwardly from the periphery substantially as is illustrated in Figs. 1

- and 3 where the collar and diaphragmare illustrated as divergin'gly spaced inwardly from their p ip ry.

This spacing of the diaphragm and collar, as' isrepresented in Fig. l, responds to the gen 'eral' formation of the diaphragm and is so proportioned that thecollar-diaphragm spacing is ,substantiallyat one of the limits of the vibratory movement of the diaphragm, so; that when' the diaphragm in its vibration movement is caused to approach the collar, the space between the diaphragm and collar will be reduced to a substantially nullity. At this point of diaphragm movement, the air. column defined by the projector and collar will have been stopped; in that the diaphragm will be very close to or substantially in engagement with the inner terminating end of the bore 55 of the 001- lar. Thus, for the best results the opposing face df the collar 50 must be similarly surfaced so as to provide an annular groove 5! corresponding substantially to the outside curvature of the diaphragm corrugation 43, and this groove151 recurves 'and merges into an annular roll or ridge 58 corresponding to the inner curvature of the diaphragm. corrugation 42, and-the roll 58 thence merges into the bore 55 as at 59.

The similar surfacing of the opposed parts .of the diaphragm and collar, with the location of the collar surface substantially atone of the J limits of diaphragm vibration provide for the f end of the compression stroke.

.termine whether or not the diaphragm does 210- phragm and collarat the endof the compression stroke. It is practicallyimpossible to detually engage the surface of the collar, but it can be said that the diaphragm is moved into intimately close relation with the 001131.

Subsequent reference to collar-diaphragm engagement 'is therefore'to be interpreted as meanmand.

cured, the parts are maintained in the desired ing that relatively close relation in which the diaphragm and collar surfaces are very close together, but should not be limited to a relation in which the parts are positively and firmly pressed together.

In the operation of the sounding signal, by vibration of the diaphragm it will move to and from the collar 50 stopping the air column therethrough. The movement or approach of the diaphragm with respect to the collar is in the nature of rolling engagement from the periphery toward the axis thereof, since the stiffening of the diaphragm by reason of the corrugations causes the outer portion near the corrugation 43 to first flex which allows the adjacent portion to engage the collar, the point of flexing moving inwardly toward the axis thereof as the outer portion of the diaphragm comes into engagement with the collar, and of course as the point of diaphragm flexing moves inwardly so also the area of diaphragm and collar substantial engagement, or intimately close relation is increased as the diaphragm assembly is caused to approach the mouth of the air column. However, the diaphragm is not thought to actually touch the collar at any point other than the area of the clamping portion at the periphery, otherwise an objectionable tone is produced. In other words, upon operation of the power plant, the diaphragm in its movement toward the collar rolls inwardly from the periphery toward the center thereof, and this in effect squeezes out substantially all of the air content of the space between the diaphragm and collar which must pass through the throat thereof, so that the mouth of the air column is stopped sharply, yet wholly in absence of any mechanical noise and clicking, as is usually the case with prior art devices. It is of course obvious that upon the retracting movement of the diaphragm assembly from the inner end of the air column, that a volume of air will rush in to fill the space left by the rolling away of the diaphragm from the collar and which on repeated movement of the diaphragm is driven out at the end of the subsequent compression stroke, thus setting into vibration the air column.

In Fig. 3 there is illustrated a modified form of the diaphragm collar association, in that the diaphragm there is represented by a flat plate 40a, that, like its companion member of Fig. 1, is secured to the power plant by clamping screws 45a and a collar 50a. The diaphragm is similarly secured to the armature rod 33 by a nut and washer device as at 39, and that operates to clamp it between the clamping discs 38a. as in the earlier described form. In this instance the collar 50a is of composite form, providing the diaphragm engaging portion 56a. as an annular ring within which is threaded an intermediate portion 501) having an axial boss la internally threadedto receive a threaded collar 53a'secured upon the toe 54a of the trumpet. There is also provided a. clamping nut 6| that threads upon the member 53a and abuts the end of the boss 5| afor maintaining the trumpet in its predetermined relation with respect to the said collar. This structure makes provision for varying the diaphragm-collar space as will be apparent from the structure illustrated in that the intermediate portion 50b may be threaded in or out of the ring portion 56a as option may de-.

When this adjustment has been proposition by means of a keeper plate 62 resting on the rim 56a and having a portion engaging the periphery of the intermediate portion 50b is is illustrated in the lower portion of Fig. 3, the keeper being retained in place by a cap screw 63 threaded into the said ring.

An important feature of operation of either form, or of an equivalent device, is that the envelope of diaphragm movement as respects the possible volumetric control of the chamber, shall be relatively large in volume as compared to the volume of unexpelled air remaining in the chamber at the end of the compression stroke. Both forms of the disclosure are thought to accomplish that end, and result in the rapid movement of comparatively large volumes of air so that there is a surge of air back and forth through the trumpet opening, thereby setting into audible vibration the air colunm defined by the trumpet.

While I have illustrated'only Fig. 3 as providing an adjustable collar, I do not propose to limit that type of structure to the illustration of Fig. 3, since it is apparent that the same features may apply in an instance of that kind as well.

While the form of embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

1. A sound signal comprising in combination, a motor, a trumpet, a collar supporting the trumpet, and a diaphragm actuated by the motor for stopping the inner end of said trumpet when moved to one extreme, said diaphragm and collar being under peripheral enga ement and spaced apart at the axis thereof, the diaphragm collar spacing increasing from the peripheral engaging portion toward the center, the daphragm when vibrated to stop the said trumpet substan tially eliminating any space between the collar and diaphragm.

2. A sound signal comprising in combination, a motor, a trumpet. a diaphragm and a trumpet collar set against the diaphragm edge but in creasingly spaced therefrom toward their common axis, said motor being operable to vibrate the diaphragm, and said collar-diaphragm spacing being substantially at one of the limits of diaphragm movement, whereby the diaphragm at one extreme of movement will be in substania-l surface engagement with said collar.

3. A sound signal comprising in combination, a motor, a collar support ng a trumpet. a diaphragm secured between the collar and motor and having driving engagement with the latter, means securing the dia hragm and collar in peripheral engagemen, sa'd d aphragm and collar being surfaced in mating relation at their opposing faces to form a relatively thin air space therebetween, said air space increasing in cross section from the periphery toward the center and being reduced to a substantial nullity when the dia phragm is moved to stop the air column through the collar and trumpet.

4. A sound signal comprising in combination, a motor, a rigidly supported diaphragm driven thereby, and a collar supporting a trumpet and fixed in peritheral engagement with the diaphragm edge but increasingly spaced therefrom toward the common axis thereof, said diaphragm having a central corrugated portion to stiffen the tially at one of the limits of diaphragm actuation,

said central portion so as to increase the volume of air movement, said collar being similarly surfaced to cooperate with the corrugations of said diaphragm whereby vibration movement of the diaphragm will drive out substantially all of the air from the said. space between the diaphragm and collar when thediaphragm stops the said trumpet. v.

5. A sound signal comprising in combination, a motor, a diaphragm driven by the motor, a collar supporting a trumpet and peripherally engaging the diaphragm, said collar having its inner portion increasingly spaced from the periphery toward. the center of the'diaphragm, but. sufiiciently closely related therewith to be substanwhereby the diaphragm when vibrated will be substantially in surface engagement with the collar at one extreme of its vibratory movement.

6. A sound signal comprising in combination, a magnet motor, a diaphragm supported for actuation thereby, a collar comprising a peripheral portion and an inner portion concentric therewith, a trumpet supported by said collar, means including the collar peripherally engaging the diaphragm, said collar having its inner portion.

increasingly spaced from the diaphragm toward their common center, but sufficiently closely related therewith to be substantially at one of the limits of diaphragm vibration, whereby, the diaphragm when vibrated will be substantially in sur face engagement with the collar at one extreme of its vibratory movement, and means for varying the relative position of the inner portion of the collar with respect to the diaphragm and with respect-to the peripheral portion of the collar.

7. A' sound signal comprising in combination a magnet motor, a trumpet, a collar supporting the trumpet and fixed to the magnet motor, and a diaphragm driven by the magnet motor for stopping the inner end of said trumpet upon one extreme of diaphragm movement, said collar engaging the diaphragm at its edge and being divergingly spaced therefrom so that the normal distance from the diaphragm to the collar gradually increases from the periphery thereof toward the center, the diaphragm, when vibrated to stop the said trumpet operable to squeeze substantially all of the air out of the space between the diaphragm and collar.

8. A sound signal comprising in combination, a motor, a trumpet, a collar supporting the trumpet from the motor and a diaphragm clamped" between the edge of the collar and said motor and connected to the latter for movement gage one another throughout substantially theentire surface of said collar when the diaphragm is actuated to one extreme of its movement,

whereby substantially. all of the air content of the spacing between the diaphragm and collar may be eliminated at the stopping of the trumpet by thediaphragm.

9. A sound signal comprising in combination, a magnet motor, a trumpet, a collar supporting the trumpet and securing the same to the magnet motor, a vibratile diaphragm clamped at its edge to the collar and having a driven connection with said magnet motor whereby the diaphragm'may stop the toe of the-trumpet at one extreme of its movement, said diaphragm having annular corrugati'o'ns' to substantially localize its flexing to an area near its edge, the diaphragm and the collar being normally in engagement at the periphery of the diaphragm and the diaphragm corrugation permitting the diaphragm to flex within. the peripheral engaging portion so as to approach thecollar and so as to squeeze substantially allof the air out of the space between the diaphragm and collar as the diaphragm is actuated to stop the trumpet. I

10. A sound signal comprising in combination,

a 'magnet motor, 'a trumpet, a collar supporting the trumpet and securing the same to the magnet motor, a vibratile diaphragm clamped at its edge to the collar and having a driven connection with said magnet motor whereby the diaphragm may stop the toe of the trumpet at one extreme of its movement, the diaphragm and the collar being,

' creasingly spaced toward the axis thereof, the

diaphragm when vibrated to stop the said trumpet substantially engaging the entire juxtaposed surface of said collar. i

12. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a vibratile diaphragm secured.

at its edge to the collar and annularly corrugated so as to rigidify the central portion thereof and substantially localize a flexing portion within the secured edge, annular ribs and Vales on the face of said collar adjacent the diaphragm arranged to cooperate with the diaphragm corrugations so as to be substantially in surface engagement with the same at one extreme of the diaphragm vibration, said diaphragm and collar defining an enclosed chamber of increasing cross section toward the collar aperture, and means operative from said motor to vibrate the diaphragm so that it will substantially engage the entire juxtaposed surface of the collar whereby said chamber is practically collapsed.

13. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a flexible diaphragm secured at its edge to the collar, and concentrically corrugated so as to produce a relatively rigid central portion that decreases in rigidity toward the attaching edge, means for flexing the diaphragm upon actuation of the motor, surface provisions on the opposing face of the collar member similar to the corrugations of said diaphragm, said collar having its opposed face situated substantially at .one of the limits of diaphragm fiexation, whereby the diaphragm will substantially engage the similar opposed surface of the collar member thereby practically collapsing the chamber existing between the diaphragm and collar that exists-under normal conditions.

14. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a flexible diaphragm secured at its edge to the collar, and concentrically corrugated so as to produce a relatively rigid central portion that decreases in rigidity toward the attaching edge, said collar having an opposed surface corrugated in similarity with the diaphragm and defining an air chamber therebetween of gradually increasing cross section toward the center thereof, and said surface being located substantially at one of the flexing limits of said diaphragm, means for flexing the diaphragm by the motor so that the diaphragm and collar spacing within the annular area at the diaphragm edge is done away with, in effect substantially eliminating the air chamber between the diaphragm and collar.

15. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a flexible diaphragm secured at its edge to the collar, and concentrically corrugated so as to produce a relatively rigid central portion that decreases in rigidity toward the attaching edge, said collar having an opposed surface corrugated in similarity with the diaphragm and defining an air chamber therebetween of gradually increasing cross section toward the center thereof, and said surface being located substantially at one of the flexing limits of said diaphragm, means for flexing the diaphragm by the motor so that the diaphragm and collar substantial engagement will be enlarged to include substantially the entire surface of the adjacent collar.

16. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a flexible diaphragm secured at its edge to the collar, and concentrically corrugated so as to produce a relatively rigid central portion that decreases in rigidity toward the attaching edge, said collar having an opposed surface corrugated in similarity with the dia phragm and defining an air chamber therebetween of gradually increasing cross section toward the center thereof, and said surface being located substantially at one of the flexing limits of said diaphragm, means for flexing the diaphragm by the motor so that the diaphragm will substantially engage the collar within the secured edge of the diaphragm and will progress toward the collar aperture so as to include substantially all of the opposed collar surface as the diaphragm is flexed toward one extreme of its movement.

17. A sound signal comprising in combination, a motor, a trumpet, a collar supporting the trumpet from the motor and having a peripheral diaphragm engaging portion, a diaphragm clamped between the edge of the collar and said motor and connected to the latter for movement thereby, said collar having a second portion joining the diaphragm engaging portion and that is spaced from the inner portion of the diaphragm,

stopping the inner end of said trumpet when moved to one extreme, said diaphragm and collar being under peripheral engagement and spaced apart at the axis thereof, the diaphragm-collar spacing increasing from the peripheral engaging portion toward the center, the diaphragm when vibrated to stop the said trumpet substantially engaging the entire juxtaposed surface of said collar.

19. A sound signal comprising in combination, a motor, a trumpet, an apertured collar opening into said trumpet, a vibratile diaphragm secured at its edge to the collar and annularly corrugated so as to rigidity the central portion thereof and substantially localize a flexing portion within the secured edge, annular ribs and valleys on the face of said collar adjacent the diaphragm arranged to co-operate with the diaphragm corrugations so as to be substantially in surface engagement with the same at one extreme of the diaphragm vibration, said diaphragm and collar defining an enclosed chamber of increasing cross section toward the collar aperture, and means operative from said motor to vibrate the diaphragm so that it will substantially engage the entire juxtaposed surface of the collar, whereby said chamber is practically collapsed.

20. A sound signal comprising in combination, a motor, a trumpet, a plate having an aperture leading to the trumpet, a diaphragm fixed at its edge to the edge of the plates and widely spaced therefrom at the common center, said diaphragm vibrating into such close proximity to the plate that substantially all of the air between the diaphragm and plate is squeezed out and the inner end of the trumpet is practically stopped when the diaphragm is vibrated to .one extreme of its movement, and means for vibrating the diaphragm from the said motor including a rigid connection between the diaphragm and a movable part of the motor.

21. A sound signal comprising in combination,-

a motor, a trumpet, a plate having an aperture leading to the trumpet, a flexible body, the major portion of the area of which is oscillatable in such close proximity to the plate that natural vibrations of the body are suppressed by substantial engagement between the body and plate, and so that the air column in the trumpet is substantially stopped, means supporting the flexible body in edge to edge relation with the plate to form a collapsible space between the major portion of the flexible body and the plate, and means including the motor for actuating the body to squeeze out substantially all of the air from the space between the plate and body.

22. In a sound signal of the trumpet type comprising in combination, a motor, a trumpet, a diaphragm secured at its edge and having its center operatively connected with said motor, a collar member peripherally engaging the diaphragm and having a surface juxtaposed in substantially engageable relation therewith, a trumpet opening in said collar, said diaphragm operating to stop the air column in said trumpet, and to drive substantially all of the air in the space between the diaphragm and collar through the trumpet upon its compression movement toward the collar. I

23. A sound signal comprising in combination, a motor, a diaphragm driven by the motor, means supporting the motor and the diaphragm in concentric relation, said diaphragm being corrugated concentric of the support, a collar member cooperating with the supporting means to secure the diaphragm at its edge, and having its adjacent surface formed in mating relation with the contour of the diaphragm and set at substantially the limit ofdiaphragm movement whereby said diaphragm when vibrated to approach the collar will substantially engage the entire opposed surface of said collar.

24. A sound signal comprising in combination, a motor, a trumpet, a plate having an aperture leading to the trumpet, a flexible body of air impervious material, means including the motor for oscillating the flexible body in such close proximity to the plate that the plate is substantially engaged by the flexible body, and so that natural vibrations of the body are suppressed below the audible volume and the air column in the trumpet is substantially stopped, means including the plate edge for supporting the vibratory body in spaced relation with the plate to'form a collapsible chamber therebetween, and so that during a condition of rest there will be a thin air space between the plate and body increasing in cross sectional magnitude from the edge toward and up to the plate aperture, the body at one extreme of its oscillatable movement substantially collapsing said chamber. I

- 25. In an acoustic device a diaphragm, a plate having a rim portion clamped to the edge of the diaphragm, and an inner deflected portion sloping away from the diaphragm forming a shallow chamber with the diaphragm, means vibrating the diaphragm to and from the plate and substantially collapsing the chamber at one point in the vibration of the diaphragm, said plate having an opening therein, and means including a part of the diaphragmior substantially expelling the air content from the chamber through said opening.

26. In a sound signal the combination comprising, a diaphragm, a collar peripherally engaging the diaphragm, and having its central area spaced from but located substantially at one of the limits of diaphragm vibration, a projector opening through said collar, means for vibrating the diaphragm to stop the air column through the projector, said diaphragm and collar being similarly surfaced in contour at their contiguous faces and substantially engageable throughout their opposed surfaces when the diaphragm is driven against the collar so as to reduce the space therebetween to a. nullity upon one extreme of diaphragm vibration movement. a ROBERT M. CRITCHFIELD. 

